As used today, Facebook is more marketing mechanism than social medium.
As such, the natural inclination is to divide and sub-divide its collective mass of subscribers into highly defined fragments for target marketing purposes.
While this practice has value from a business standpoint, it’s probably lacks merit for better social development in society-at-large. It creates, as you aptly described it, echo chambers; instead of promoting the cross-fertilization of ideas and mutual understanding (mixed communities in which people learn and live together), it coaxes people to withdraw into safe havens of their own like-minded kind (what is known as group-think). It’s an anti-social outcome for the sake of a business-driven intent.
This creates a form of large-scale cognitive dissonance. From a social good standpoint, there’s a tendency to want to believe it can help us, in an altruistic way, to build toward a collective social consciousness. From a business standpoint, it more often does the opposite and yields a negative net social effect.
I don’t blame Facebook for this collateral social damage, because it’s just a tool. The users have made their choice for how to use it. Anyone who uses it while thinking about it in a social good context — or as social media, would probably be well-advised to re-consider their perspective. It’s safer, both personally and professionally, to recognize it as a business tool and nothing more.